General communications with, and data from, the two rovers are transmitted either directly back to Earth (about 20% of the total) or are related to Odyssey, MGS, or Mars Express, according to this scheme:

MER-1 (also referred to as MERA), nicknamed "Spirit", was launched towards Mars on June 10, 2003. MER-2 (MERB), called "Opportunity", was launched on July 7, 2003. Here is a plot of the landing sites for Express's Beagle 2 (failed) and the two MERs set on a backdrop of the Nars surface in a global projection:

Spirit arrived at the Red Planet on time and sent its Rover to the surface with a successful landing at 8:35 PM (PST) on January 3, 2004. Spirit set down in the expansive Gusev Crater, which was postulated to contain frozen water inside. Opportunity (discussed on the next page) went to Meridiani Planum and landed among rocks rich in hematite (iron oxide) which can be associated with water-rich conditions. Here are both landing sites as mapped using Mars Orbiter data:

The Gusev crater, which is about 200 km wide (125 miles), appears as follows in a Mars Orbiter image:

A map of elevations in and around the crater appears here:

The rationale behind choice of Gusev as one of the MER sites was based on several criteria: 1) the apparently smooth, flat surface is typical of lake beds (Gusev was postulated to have been filled with water at some time); 2) a long 800 km [500 mile], sinuous channel, Na'adim Vallis, seems to have breached the crater walls; 3) faint layering is visible in high resolution images of the crater walls. Another constraint on site selection was to have it about 180° from the second rover site, so that one site faced Earth during an approximately 12 hour period (allowing signal transmission) and the second site could broadcast during the other 12 hour time frame (Mars' day is a little longer than 24 hours).

This MOC image shows the initial touchdown point at Bonneville Crater and the subsequent traverse path followed by Spirit as it eventually reached and climbed the hills to the southeast.

Soon after touchdown, Spirit was located by the Mars Global Surveyor, in this MOC image, which also shows what is interpreted to be surface disturbances caused by the bouncing (an estimated 28 times) of the airbag-cushioned rover as it moved up and down after first impact:

The Spirit Rover took a picture of itself using the Navigation camera mounted on a pole at the top. Here it is sitting on the surface:

Pre-landing interpretation of the Gusev floor's geology led to the assumption that this crater;s floor was once covered with water, i.e., it was an ancient lake. Thus rocks within, and perhaps the soil, might show some signs of life, since water normally is a prerequisite. The first color image produced by Spirit does not show any direct evidence of lake deposits; in fact, it closely resembles surfaces imaged by the Vikings and Pathfinder:

None of the rocks looked at during the early traverses (examples just below) seem to show lacustrine layering (lake bed deposits on Earth often show narrow bands of alternating composition - light/dark/light/dark - indicating seasonal changes). First impressions are that these rocks are similar to basalts, which may be the dominant rock type on Mars. Even if Gusev does contain lake deposits, many (most?) of the surface rocks at the site could be ejecta from impacts beyond the crater that penetrated into basaltic bedrock. The hope for confirming lake beds rests mainly with the presence of their characteristic rocks as ejecta from local smaller craters within Gusev Crater. These rocks may be lighter in color/tone; several lighter rocks have been spotted so far

While waiting to deploy the Rover on its first excursion, sensors on Spirit acquired various imagery and data. Look first at a 360° panorama around the touchdown site, made using more than 200 images (scroll horizontally to see the full extent):

The surface within the crater is not all flat. Low hills are seen in the background. These initially were called the East Hills but since have been renamed the Columbia Hills. Chances are the hills themselves are upthrown floor or interior peak rocks made during Gusev's impact formation and would likely be older martian crust (basaltic) rather than lake bed deposits. The seven crests along the hills were remaned after the 7 astronauts who died in the Columbia Shuttle disaster. Here they are, seen in a panoramic view from Spirit:

The onboard MTES (Mini Thermal Emission Spectrometer) has produced spectra for the area close to the instrument. Both silicates and CO2 were detected; the latter is not in the rock/soil but in the martian atmosphere. Here are the plots.

Spirit took off on its first excursion on January 14, 2004. The planned traverse is shown below. The first major objective was the impact crater named Bonneville which has excavated the possible lake beds to a depth of 30 m. Rocks enroute were examined to see if they are compatible with the types formed in lakes. The dark bands in the image probably represent the tracks of dust devils that remove surface fine particles exposing darker rock below.

Using a special technique, the Mars Global Surveyor was able to image the Bonneville scene at 0.5 m resolution, sufficient to pick out the landing site platform, the travel track to Bonneville, and the rover itself on the edge of the crater ejecta slope (note that the surrounding blanket is lighter in tone than the general martian surface traversed; this is not an indication of a compositional change but probably results from the higher reflectivity of the ejecta associated with the crater).

The trip to this crater (whose dark surface suggests basaltic rock) took time, as experience was gained in navigating the Rover from millions of kilometers away. The first stop was an analysis of a football-sized rock nicknamed "Adirondack" (an American Indian name for "They of the Great Rocks"). It is believed to be volcanic (likely, basalt) in nature. Here is a natural color view:

Enroute to Adirondack, Spirit's analysis of nearby soil showed the presence of olivine, a greenish magnesium-iron silicate that is a constituent of basalt. The mineral probably is present as loose grains blown to the site by winds.

This view shows the extended robotic arm (Instrument Deployment Device: IDD) containing several of the analytic instruments up against the Adirondack rock:

On January 21, 2004 Spirit went temporarily into a "repair mode", as it ceased to send back data although ground controllers at JPL could still communicate with it. Thereafter, several small streams of data were received but were largely unusable. The problem has been diagnosed as a software misfunction in which the storage device simply ran out of memory when commands were misinterpreted, and caused file corruption. The "flash memory" has since been purged and debugged, and functions reset. Mission Control is cautiously optimistic that the Spirit Rover can continue its excursions and collection of scientific data indefinitely and that the same problem in Opportunity can be avoided by modifying its software.

On January 30, Spirit returned a spectral analysis of the Adirondack rock, confirming it to be basaltic in composition, as seen in this spectral curve:

Then, on February 4th Spirit began a visual inspection of the Adirondack surface. On a "hunch" the operational team suspected there might be a thin coating of dust on that surface. Attached to the RAT (Rock Abrasion Tool) is a wire brush. The rover was instructed to brush off a small patch on the surface, with this result:

The dust coating (almost certainly hematite) was less than a millimeter thick; it may be adhering by electrostatic forces. This dust appears to be widespread across Mars' surfaces and may be compromising some of the compositional data obtained from the Viking sites and from orbit (e.g., Odyssey and MGS). When the dark patch was examined by the Microscope Imager (MI), this image was obtained - it appears consistent with a rock having a texture typical of basalt.

Adirondack is the first rock to be drilled into, using the RAT (Rock Abrasion Tool). Here is the rock showing the circular cutting outline:

As Spirit moved on, it spied one unusual rock lying as float on the surface. This rock (below) is splitting into thin flaky layers or chips. It resembles a fissile shale on Earth but is richer in iron than most shales (but there are red shales). Such layering is usually a consequence of continuing deposition in a fluid medium, usually water.

Spirit's Microscope Imager looked at tiny dunes (sand ripples) as it traversed towards Bonneville crater. The resulting grains on a ripple crest range from angular to round (and may relate to the spherules found at the Opportunity site?):

On March 10th, Spirit reached the rim of Bonneville crater (about 200 meters in diameter) and looked in to see the interior:

At this resolution, no obvious layers of rock appear within the crater (some blocks in the far slope may be loose or may be outcrop in otherwise loose fill).

While in the rim, Spirit took a pan camera image of a scuffed (scraped) surface. It shows that darker material lies just underneath, which may be basaltic sand covered by hematitic dust:

Spirit's Microscope Imager has also examined a sand deposit on the Bonneville Crater rim. After it was scraped, this image shows a crust of lighter-tone granular material overlying a darker substrate:

The lower zone contains rounded particles up to about 0.5 cm in diameters mixed with much smaller grains (~50µm).

As it moved from Bonneville, Spirit spied a large rock which was named Mazatzal. It was partially covered by windblown sandy soil

Several RAT grinds penetrated this rock. It revealed at least two very thin layers which have different emissivities, as indicated in this plot:

The pre-grind surface is darker in the visible; after grinding the rock underneath is brighter. Thus, this basalt block has experienced some process that darkens its outermost layer (or deposits it). An analogy found in Earth rocks is the formation of "desert varnish" which is blacker because of MnO2; but this composition has not been confirmed for Mazatzal. Part of that dark surface remains in the RAT hole seen in the next image:

The presence of a cross-cutting vein filled with light material has caused excitement among the Spirit scientists. Although its composition is as yet unknown, the filling is postulated to be the result of deposition of mineral(s) of secondary origin that precipitated out of a hydrous solution that came from beneath the basalt before it was released (probably by impact) from its bedrock. This is believed to be an indicator of hydrothermal activity after lava emplacement and cooling. (Groundwater circulation is another suggestion.) This veining in this and another Gusev crater rock is the best evidence so far for some water having affected the bedrock present in the crater.

However, APXS data have confounded interpretations of whether a water body at least briefly covered the Gusev surface after the basalt blocks were brought to the surface. Examine this plot of Magnesium oxide (MgO) content versus SO3 content of the very thin veneer covering several Spirit rocks including Mazatzal:

The three large symbols on the ordinate are from the rock materials beneath the veneer. No free sulphur is present, indicating its absence in the basalt. However, fresh, brushed, and slightly RATed surface all have evidence of sulphur, shown as a trioxide but without any specific mineral type being identified (still unknown). Speculation has it that this is some type of coating that may have been associated with precipitation of sulphur-bearing minerals on the surface and in the soil. A similar pattern is found for the data that indicate Chlorine (Cl) to be present in the veneer and to increase in proportion to the rise in SO3.

Both Chlorine and Bromine are found in coatings on rocks from both the Spirit and Opportunity sites. On Earth Chlorine and Bromine tend to increase linearly in evaporites. The situation is somewhat different in rock coatings from both MER sites, as shown in this general plot of the concentrations of these two co-varying elements.

Summing up the first year, the story at the Gusev Spirit site was this: The only rock type firmly identified is basalt; hematite is of the red variety; although several rocks may contain hydrous mineral(s), no other signs of lake water or water-related sediments, including strata of any kind, had as yet been detected; however, hydrothermal or groundwater activity from subsurface sources has been verified. Coatings containing Cl, Br, and SO3 remain enigmatic. One hypothesis is now more favored: the coating(s) may be dust that has been cemented as a crust onto the surface with water from an atmospheric source. In retrospect, the Gusev site does not now seem to have had a lake or extensive sediments, still, water activity is probable but not to the extent favoring life, and the sinuous channel may not be fluvial in nature. The possibility that Gusev contains sediments below a basalt layer as established from the composition of the rocks scattered about the surface, the latter thus being a later emplacement of covering lava, cannot yet be discounted but later cratering into the basalt has not penetrated to the depths of these possible deposits.

Spirit set out in late 2004 for the Columbia Hills and reached them in early 2005. In this view of these Hills hints of layering can be seen through its pan camera. Enroute, Spirit has examined other surface rocks, especially those lighter in tone, as seen in these views, to test the surface coating hypothesis or find alternate explanations:

As the Columbia Hills were approached, possible outcrops were detected:

As seen in this MOC image from MGS, the Columbia Hills appear as below.